I think the real reason why I constantly check Facebook is to see, from the number of notifications I have, which of my friends acknowledges and misses me.
But Facebook is a horrible representation of real life. Even if you're thinking about one of your friends, the chance that you'll write something on their wall or send them a message or even comment on something of theirs is very low. I know this because I do it all the time - or rather, I don't do it.
So in the end, logging on yet again into Facebook just results in my feeling sad and ignored and forgotten. But no matter what I do, even if I deactivate my profile, I can't fight the urge to not log on. Circle of bad addiction, I guess.